The present invention generally relates to electrophotography and more particularly, to a transfer type electrophotographic copying apparatus with variable magnification, which is so arranged that, with scanning speed of an optical scanning system being altered according to copying magnifications, copy paper sheets are transported at a predetermined speed for copying at varied magnifications.
Generally, in a transfer type electrophotographic copying apparatus with variable magnification, there has been employed a system in which circumferential speed of a photosensitive member or photoreceptor and speed for transportation of copy paper sheets are maintained to be constant, while scanning speed of a scanning system for an original document is altered according to the copying magnifications as disclosed, for example, in U.S. Pat. No. 4,126,389. In the conventional system as described above, it is so arranged that copy paper sheets are transported from a predetermined stand-by position to an image transfer station so that a leading edge of an electrostatic latent image formed on the photoreceptor is registered or coincides with a leading edge of the transported copy paper sheet at the image transfer station, with a timing for starting transportation of the copy paper sheet being provided through signal from the original document scanning system.
In the known arrangement as described above, however, since the scanning speed is also altered upon variation of the copying magnification, time required from starting of the scanning up to emission of the copy paper transporation starting signal become undesirably different for each different copying magnification, thus resulting in a deviation in the synchronization between the leading edge of the electrostatic latent image and that of the copy paper sheet.
In order to eliminate the deviation in the synchronization as described above, there has also been conventionally proposed, for example, in U.S. Pat. Nos. 4,126,389 and 4,217,052, a system in which a plurality of detecting means corresponding to the copying magnifications are provided in the scanning system, and upon selection of a certain magnification, signal from the detecting means corresponding to the selected magnification is utilized as the copy paper sheet transportation starting signal. In the prior art system as described above, since the timing for starting of the copy paper sheet transportation may be readily varied through alteration of the installed position of the detecting means, adjustments can be simply made even when there are deviations in the response time and physical positionings of respective parts according to individual copying apparatuses, but in the case where a variety of copying magnifications are required, the number of detecting means is undesirably increased, with consequent high cost and complication of constructions.
In another conventional system disclosed, for example, in U.S. Pat. No. 4,129,377, attention is directed to the fact that, although scanning speed of the scanning system is varied according to alterations of the copying magnifications, circumferential speed of the photoreceptor is constant at all times, and thus, the time required from starting of light image projection of the leading edge of an original document onto the photoreceptor up to arrival of the leading edge of its electrostatic latent image at the image transfer station, is also constant at all times. More specifically, if the arrangement is so made that, by providing a detecting means for detecting that the scanning system has reached the original document leading edge exposure starting position common to any of the copying magnifications, the copy paper sheet is started to be transported from the stand-by position towards the image transfer station after a predetermined period of time not dependent on the copying magnifications, from emission of the signal by said detecting means, the leading edge of the electrostatic latent image on the photoreceptor is registered with the leading edge of the copy paper sheet at all times for any copying magnification. In the known system as described above, only one detecting means is required for the scanning system, and therefore, constructions may be simplified as compared with the system previously described, with a consequent reduction of cost, while on the other hand, there is such a disadvantage that, even if the detecting means is correctly aligned with the original document leading edge exposure starting position, in the case where deviations exist in the response time and physical positionings of respective parts, the time from emission of the detecting signal to starting of transportation of the copy paper sheet must be subjected to fine adjustments. Generally, the time as described above is composed as a program (i.e. ROM or read only memory) of a microcomputer or other digital circuits. In such a case, it is extremely difficult to adjust said time with respect to individual copying apparatuses. In other words, even a timer composed of an electrical circuit is difficult to be adjusted, and moreover, in the case where the timer value is set as a program of a microcomputer, the microcomputer itself must be replaced.